As far as I know you cannot do that using the old command prompt. However, you can find your key if you use powershell. In order to find it, you'll have to run the following script (it suffices to copy & paste it in powershell and then press enter) :
# create table to convert in base 24
$map="BCDFGHJKMPQRTVWXY2346789"
# Read registry Key
$value = (get-itemproperty "HKLM:\\SOFTWARE\Microsoft\Windows NT\CurrentVersion").digitalproductid[0x34..0x42]
# Convert in Hexa to show you the Raw Key
$hexa = ""
$value | foreach {
$hexa = $_.ToString("X2") + $hexa
}
"Raw Key Big Endian: $hexa"
# find the Product Key
$ProductKey = ""
for ($i = 24; $i -ge 0; $i--) {
$r = 0
for ($j = 14; $j -ge 0; $j--) {
$r = ($r * 256) -bxor $value[$j]
$value[$j] = [math]::Floor([double]($r/24))
$r = $r % 24
}
$ProductKey = $map[$r] + $ProductKey
if (($i % 5) -eq 0 -and $i -ne 0) {
$ProductKey = "-" + $ProductKey
}
}
"Product Key: $ProductKey"